Polyethylene terephthalate filaments of high strength are well known in the art and commonly are utilized in industrial applications. These may be differentiated from the usual textile polyester fibers by the higher levels of their tenacity and modulus characteristics, and often by a higher denier per filament. For instance, industrial polyester fibers commonly possess a tenacity of at least 7.5 (e.g. 8+) grams per denier and a denier per filament of about 3 to 15, while textile polyester fibers commonly possess a tenacity of about 3.5 to 4.5 grams per denier and a denier per filament of about 1 to 2. Commonly industrial polyester fibers are utilized in the formation of tire cord, conveyor belts, seat belts, V-belts, hosing, sewing thread, carpets, etc.
When polyethylene terephthalate is utilized as the starting material, a polymer having an intrinsic viscosity (I.V.) of about 0.6 to 0.7 deciliters per gram commonly is selected when forming textile fibers and a polymer having an intrinsic viscosity of about 0.7 to 1.0 deciliters per gram commonly is selected when forming industrial fibers. Both high stress and low stress sprinning processes heretofore have been utilized during the formation of polyester fibers. Representative spinning processes proposed in the prior art which utilize higher than usual stress on the spin line include those of U.S. Pat. Nos. 2,604,667; 2,604,689; 3,946,100; and British Pat. No. 1,375,151. However, polyester fibers heretofore more commonly have been formed through the utilization of relatively low stress spinning conditions to yield a filamentary material of relatively low birefringence (i.e. below about +2.times.10.sup.-3) which particularly is amenable to extensive hot drawing whereby the required tenacity values ultimately are developed. Such as-spun polyester fibers commonly are subjected to subsequent hot drawing which may or may not be carried out in-line when forming textile as well as industrial fibers in order to develop the required tensile properties.
Heretofore high strength polyethylene terephthalate fibers (e.g. of at least 7.5 grams per denier) commonly undergo substantial shrinkage (e.g. at least 8 percent) when heated. Also heretofore, when such polyester industrial fibers are incorporated in a rubber matrix of a tire, it has been recognized that as the tire rotates during use the fibers are sequentially stretched and relaxed to a minute degree during each tire revolution. More specifically, the internal air pressure stresses the fibrous reinforcement of the tire, and tire rotation while axially loaded causes repeated stress variations. Since more energy is consumed during the stretching of the fibers than is recovered during the relaxation of the same, the difference in energy is dissipated as heat and can be termed hysteresis or work loss. Therefore, significant temperature increases have been observed in rotating tires during use which are attributable at least in part to this fiber hysteresis effect. Lower rates of heat generation in tires will lower tire operating temperatures, maintain higher modulus values in the reinforcing fiber, and extend the life of the same through the minimization of degradation in the reinforcing fiber and in the rubber matrix. The effect of lower hysteresis rubbers has been recognized. See, for instance Rubber Chem. Technol., 45, 1, by P. Kainradl and G. Kaufmann (1972). However, little has been published on hysteresis differences in reinforcing fibers and particularly hysteresis differences between various polyester fibers. See, for instance, U.S. Pat. No. 3,553,307 to F. J. Kovac and G. W. Rye.
In commonly assigned U.S. Ser. Nos. 735,849, and 735,850 (now U.S. Pat. No. 4,101,525), filed Oct. 26, 1976, are disclosed a process for forming polyester filaments of high strength having an unusually stable internal structure which particularly are suited for use in industrial applications at elevated temperatures, and the resulting product.
It is an object of the present invention to provide an improved process for the production of high performance polyester fibers of high strength which incorporates an initial draw step capable of facilitating a savings of energy when compared with typical polyethylene terephthalate drawing procedures of the prior art.
It is an object of the present invention to provide an improvement in the process of U.S. Ser. No. 735,849, filed Oct. 26, 1976, which has been found to impart additional stability and reliability to the overall process.
It is a further object of the present invention to provide an improved process for the production of high performance polyester fibers of high strength which incorporates a first draw step which is carried out in a first draw zone provided at a temperature below the glass transition temperature of the as-spun filamentary material in the surprising absence of substantial neck drawing.
These and other objects will be apparent to those skilled in the art from the following description and appended claims.